1. Field of the Invention
This invention relates generally to superconducting analog-to-digital converters and, more particularly, to a superconducting analog-to-digital converter having an error correction system for correcting a digital output signal based on non-linearities in a primary quantizer.
2. Discussion of the Related Art
High-performance analog-to-digital (A/D) converters are required in a variety of commercial and military electronic devices. For example, digital mixers rely on accurate front-end digitization of radio frequency signals with high dynamic range, wide bandwidth, and high linearity. Furthermore, in order to detect weak signals in the presence of interference, A/D converters must be able to handle a wide range of signals simultaneously. Systematic non-linearities from an input signal to a digital output signal are particularly troublesome and give rise to harmonics and intermodulation artifacts. Two of the more important measures of an A/D converter""s performance are its speed, as measured by the number of samples converted per second, and resolution, as measured by the smallest increment of change that can be detected in an analog input signal.
Numerous superconducting A/D architectures have been proposed and built, including counting type, sigma-delta, and flash designs. In fact, superconducting technology is particularly well suited to performing high-speed, high-resolution A/D conversion largely because of Josephson junctions. Josephson junctions are the basic switching elements utilized in superconducting electronic devices, and possess a unique combination of speed, sensitivity, and periodic response characteristics.
The Josephson junction is a simple switching device having a very thin insulating layer sandwiched between two superconducting electrodes. When current applied to the Josephson junction is increased above the critical current of the junction, the device is switched from a superconducting zero-voltage state to a resistive voltage state. Because this switching operation can occur in as little as a few picoseconds, the Josephson junction is truly a high-speed switching device. In a superconducting A/D converter, one or more of the Josephson junctions are combined with one or more inductive loads to perform a logic circuit.
Counting-type superconducting A/D converters have demonstrated promising results, with excellent bandwidth, resolution, and signal-to-noise ratio (SNR) at high sampling (integration) rates. The total accuracy of counting-type superconducting A/D converters is tallied on a signal-to-noise and distortion (SINAD) ratio. SINAD accounts for both noise effects (SNR) and systematic non-linearities in the A/D conversion. A potential area for improvement in this technology relates to the non-linearities that limit total superconducting A/D performance for large signals.
A typical superconducting A/D converter has a quantizer followed by a single-flux-quantum (SFQ) counter. The quantizer uses Josephson junctions to generate SFQ pulses. The generation rate of SFQ pulses is exactly proportional to the voltage of the analog input signal. The counter counts the number of pulses received over a given time period. The result is a digital output signal representing the average voltage of the analog input signal. Problematic non-linearities are commonly due to the quantizer. Specifically, although fundamental physics dictate that the conversion of voltage to SFQ pulses is perfect (a constant of proportionality of 4.83xc3x9710+14 pulses/volt-second), any non-linearity in the current-voltage transfer characteristic leads to a corresponding non-linearity in the current-pulse behavior of the quantizer.
In fact, experimentally, the assemblage of resistors, inductors, and Josephson junctions making up the quantizer acts somewhat as a non-linear resistor. As already noted, counting with the SFQ counter for a fixed time interval converts the pulse frequency to a digital word representing the average voltage over the time interval. Thus, the non-linearities in the quantizer are reflected at the output of the SFQ counter. In fact, the non-linear current-voltage characteristic of the quantizer also contributes to spurious signal generation in the converted spectrum. It has been shown that a single tone input applied to the A/D converter produces measurable harmonic response at integer multiples of the input frequency. It is therefore desirable to provide a superconducting A/D converter capable of correcting the digital output signal based on the non-linearities of the quantizer.
In accordance with the teachings of the present invention, a superconducting A/D converter is disclosed. In one embodiment, the converter includes a primary quantizer, a primary SFQ counter, and an error correction system. The primary quantizer generates primary SFQ pulses based on the voltage of an analog input signal. The primary SFQ counter tallies the primary SFQ pulses into a digital output signal based on a frequency of the primary SFQ pulses. The error correction system corrects the digital output signal based on the analog input signal and the primary SFQ pulses. Using the primary SFQ pulses to correct the digital output signal allows the converter to take into account the non-linearities of the primary quantizer. Linearity in the converter will improve overall digital system sensitivities.
Further in accordance with the present invention, a method for converting an analog input signal into a digital output signal is disclosed. The method includes the step of generating primary SFQ pulses based on the voltage of the analog input signal. The primary SFQ pulses are converted into the digital output signal based on a frequency of the primary SFQ pulses. The method further provides for correcting the digital output signal based on the analog input signal and the primary SFQ pulses.
In another aspect of the invention, a method for correcting a digital output signal of a superconducting A/D converter based on an analog input signal and primary SFQ pulses is disclosed. The method includes the step of generating an analog error signal representing a difference between the analog input signal and the primary SFQ pulses. A digital error signal is then generated based on the analog error signal. The method further provides for subtracting the digital error signal from the digital output signal.
Additional objects, features and advantages of the present invention will become apparent from the following description and the appended claims when taken in connection with the accompanying drawings.